Background and overview[1]
Tungsten phosphide (WP) catalyst has received widespread attention from researchers at home and abroad because of its excellent hydrorefining activity and excellent resistance to sulfur poisoning. Tungsten phosphide is a phosphide-rich compound with similar physical and chemical properties to nitrides and carbides, but their crystal structures are very different. In the WP crystal, the P-W bond length is 0.246nm, and its structure is determined by the parameters a0=0.5 Clariant pigment 731nm, b0=0.3248nm, c The unit cell of 0=0.6227nm is formed orthogonally, so the structure is complex. It can be considered as the smallest structural unit of a triangular prism formed by metal atoms. The internal voids of the triangular prism are composed of equivalent P surrounded by electrons with different directions. Filled with atoms, these triangular prism units are combined to form the WP lattice. The structure of phosphide is similar to that of sulfide, but there is no layered structure and it is a chemical conductor. Since P atoms occupy the internal gaps of the triangular prism, it is easier to access the active edge sites during the catalytic reaction, thus exposing more active centers and possessing high catalytic activity.
Preparation method of tungsten phosphide catalyst[1]
The phosphorus sources for preparing WP catalysts are mainly phosphates, phosphites, elemental phosphorus, etc., and the tungsten sources are mainly tungstate or tungsten oxide. Preparation methods include temperature programmed reduction (TPR) method, hydrogen plasma reduction (PR) method, low-temperature thermal decomposition method, etc.
The preparation process of the TPR method can be simplified to reduction-passivation-reduction, that is, tungsten salt and ammonium phosphate salt are used to synthesize the oxide precursor, and then the catalyst is prepared through programmed temperature reduction. And because the phosphide is reactive in nature and reacts violently with oxygen or water, the catalyst needs to be passivated in a low concentration of O2 before being exposed to the air. Before the hydrogenation reaction is carried out, the passivated phosphide needs to be heated again and reduced to remove the surface. Passivation layer can be used. The TPR method has the advantages of not requiring expensive and highly toxic chemical reagents and high-pressure reaction conditions, and the process is relatively simple. It is currently the most widely used WP catalyst preparation method.
PR method refers to the inelastic collision of high-energy electrons with H2 molecules at room temperature to produce excited state H atoms with high reducing power. This active species reduces the catalyst precursor to metal at low temperatures. Phosphide.
The low-temperature thermal decomposition method is a method developed in recent years. It uses sodium hypophosphite and sodium tungstate as precursors. They are dissolved in deionized water and then recrystallized to obtain a mixed precursor. WP was synthesized by simple low-temperature thermal treatment of the precursor under an inert gas protective atmosphere.
